Resin based dental restorative materials are becoming the material of choice by dentists and patients due to desirable aesthetic properties. However, one serious drawback associated with resin based restorative material is significant polymerization shrinkage when the material undergoes the setting reaction. Polymerization shrinkage, in turn, causes gap formation between the restoration and tooth, leading to microleakage, secondary caries or loss of restoration. Polymerization shrinkage is a result of converting the C═C double bonds of low molecular weight monomers to C—C single bonds of crosslinked polymers during the setting reaction.
Usually, a low viscosity di-functional monomer such as TEGDMA (triethylene glycol dimethacrylate) or HDDA (1,6-hexanediol diacrylate) is added as a diluent to a viscous resin such as Bis-GMA (2,2-bis[4-2(-hydroxy-3-methacryloylpropoxy)-phenyl]-propane)) to form a resin mixture so that reinforcing fillers can be more easily incorporated into the resins. However, because those low viscosity di-functional monomers have a rather low molecular weight, significant polymerization shrinkage results when compositions containing those diluent monomers are polymerized.
Various attempts have been made to utilize liquid crystal monomers to reduce polymerization shrinkage (Mol. Cryst. Liq. Cryst., 11, p385, 1970; J. Polym. Sci., A-1, 9, p1893, 1971). Most nematic liquid crystalline monomers do not polymerize efficiently and, as a result, a low degree of conversion is obtained. Qian and Litt in Contemporary Topics in Polymer Science, (Vol. 7, p361, 1992) disclosed highly smectic liquid crystal diacrylate monomers based on biphenyl mesogen; very low shrinkage was obtained. Anderson et al. (Macromol. Symp. p339, 1994) disclosed smectic and nematic liquid crystal divinyl ether monomers based on bishydroxybenzoate mesogen. However, most liquid crystalline monomers have a high liquid crystal transition temperature, high viscosity, and/or low translucency, and thus are not suitable for dental use.
Another approach to reduce polymerization shrinkage is to increase the molecular weight of the monomer and make the monomer molecule larger. However, when higher molecular weight analogues of TEGDMA, such as PEGDMA (polyethylene glycol dimethacrylate) are used, a significant reduction in mechanical strength is observed due to reduced crosslinking density of the resin matrix. Most high molecular weight mono-functional and di-functional diluents would encounter the same problem. U.S. Pat. No. 6,030,606 disclosed the use of a highly ethoxylated bisphenol A dimethacrylate (6 moles of ethylene oxide per molecule) for incorporation in dental resin mixtures for reducing polymerization shrinkage.